mu4e-drafts: improve mu4e--fcc-path

Reworked from the pull #2698 for the new branch.

mu4e/mu4e-draft.el

@@ -249,17 +249,19 @@ base filename without any Maildir decoration.
 Returns the path for the sent message, either in the sent or
 trash folder, or nil if the message should be removed after
 sending."
-  (when-let ((sent-dir
-          (pcase mu4e-sent-messages-behavior
+  (let* ((behavior
+          (if (functionp mu4e-sent-messages-behavior)
+              (funcall mu4e-sent-messages-behavior) mu4e-sent-messages-behavior))
+         (sent-dir
+          (pcase behavior
             ('delete nil)
             ('trash (mu4e-get-trash-folder parent))
             ('sent (mu4e-get-sent-folder parent))
-            ((pred functionp) (funcall mu4e-sent-messages-behavior))
             (_ (mu4e-error "Error in `mu4e-sent-messages-behavior'")))))
+    (when sent-dir
          (mu4e-join-paths
           (mu4e-root-maildir) sent-dir "cur"
-     (format "%s%s2,S" base-name mu4e-maildir-info-delimiter))))
-
+          (format "%s%s2,S" base-name mu4e-maildir-info-delimiter)))))
 
 
 (defconst mu4e--header-separator